1. Field of the Invention
The present invention relates to identifying formants as resonance frequencies of voice, and in particular to a formants extracting method capable of precisely identifying formants with less computational complexity.
2. Description of the Related Art
Generally, in order to identify formants as resonance frequencies of voice, a spectral peak-picking method for searching a maximum point in a linear prediction spectrum or a cepstrally smoothed spectrum has been largely used. However, because two formants are located closely to each other in most cases, they are shown as one maximum value in the spectrum. In the spectral peak-picking method, although a sufficiently large degree is given to an FFT (fast fourier transform) in order to obtain the spectrum, it is difficult to extract the formants accurately in a frequency region.
To solve the problem, methods for calculating a root in a prediction error filter by using a linear prediction coefficient have been presented. Among them a method for obtaining a root by using a roots extraction method and Cauchy's integral formula presented by R. C. Snell is representative.
In the roots extraction method, a short-time signal is obtained by multiplying either a Hamming window, a Kaiser window or the like by an appropriate section (approximately 20 ms˜40 ms) of a voice signal as occasion demands, a linear prediction coefficient and a prediction error filter are obtained from the short-time signal, a zero is obtained from the prediction error filter, and formants are obtained by using an equation of
  F  =                    f        s                    2        ⁢        π              ⁢                  θ        0            .      Herein, θ0 is a phase of a zero, fs is a sampling-rate of a signal, and F is a formant to be obtained. The roots extraction method is superior to the spectral peak-picking method in the analysis capacity aspect; however, it is impossible to set a definite reference for judging whether actually obtained roots are directly related to formants. In addition, because the roots extraction method has high computational complexity and low precision, it has not been widely used.
The method presented by R. C. Snell is for repeatedly searching a region in which a zero exists in a z-domain by using Cauchy's integral formula. Using this method, computational complexity and precision are improved in comparison with the roots extraction method. However, because a reference for judging whether an actually obtained root is directly related to formants is not represented, reliability is accordingly low.
Therefore, because the conventional methods for obtaining formants have lower analysis capacity, reliability, precision and/or greater computational complexity, it is difficult to analyze formants precisely.